1. Field of the invention
The present invention relates to a method for impregnating a die-cast article with a sealant and an impregnation apparatus for implementing this method. More particularly, the present invention relates to an improved impregnation method and apparatus in which the sealing property and sealing yield are enhanced.
2. Description of the Prior Art
Die-cast articles inevitably include cast defects. When die-cast articles are to be used as pressure-proof and gas-tight articles, such as housings of swash plate-type compressors, their cast defects, which are acceptable if they are used as nonpressure-proof and/or non-gas-tight articles, must be sealed since the pressure leaks through the cast defects. In order to seal the cast pores of die-cast articles, the cast pores are impregnated with a sealant.
The sealant includes inorganic material, such as water glass, and organic material, such as a resin. Acrylic resin is frequently used as a resin since it has a good sealing property and is inexpensive. "Sealing property" herein means the ability of a sealant to gas-tightly seal the cast defects, which, in general, are very minute. In order to test the sealing property of a swash plate-type compressor housing made of a die-cast article, a pressure of 45 kg/cm.sup.2 (441 MPa) is applied to the housing, previously subjected to sealant impregnation. If pressure leakage occurs, the sealing property is unacceptable. This test method is carried out under very severe conditions since a swash plate-type compressor housing must have a considerably strict pressure-proofness and air tightness.
A prior art is explained with reference to FIG. 1.
In FIG. 1, a workpiece (not shown) is placed within an impregnation vessel 1 and then the inner space of the impregnation vessel 1 is evacuated to create a vacuum. A sealant storage vessel 2, which is open to the ambient air, is communicated with the impregnation vessel 1 via a channel equipped with valves 3. The valves 3 are opened so that the sealant flows from the sealant storage vessel 2 into the impregnation vessel 1. Subsequently, the valves 3 are closed, and the pressure within the impregnation vessel 2 is enhanced so as to impregnate the workpiece with the sealant. Then the valves 3 are again opened so that the sealant reflows from the impregnation vessel 1 into the sealant storage vessel 2. The impregnation vessel 1 is opened to the ambient air and the impregnated workpiece is withdrawn from the impregnation vessel 1 and is subsequently washed and heated to harden or dry the sealant. The prior art described with reference to FIG. 1 cannot attain a high sealing property which is desired for a pressure-proof and gas-tight article bacause during the flow of the sealant from the sealant storage vessel 2 into the impregnation vessel, air is dragged into the sealant and bubbles are formed.
More in detail, since the sealant storage vessel 2 is opened to the ambient air and the pressure in the impregnation vessel 1 is adjusted to low to degas the air contained in the cast defects of the workpiece, a great pressure difference is created between the two vessels when the sealant starts to flow from the sealant storage vessel 2 into the impregnation vessel 1. The sealant flow is, therefore, vigorous in a turbulent flow which drags air thereinto, causing bubbles to form.
According to another prior art, a workpiece is placed in a pressure kettle and the pressure of the kettle inner space is decreased to a vacuum so as to degas the air from the cast defects of the workpiece. Then the workpiece is subjected to sealant impregnation to fill the degassed cast defects with a resin sealant. Next, the workpiece is subjected to hydroextraction and then is rinsed with cold water so as to remove the excessive resin sealant which covers the workpiece in a film form. Finally, polymerization of the resin is carried out in boiling water. In this prior art, the fraction defective, which is unacceptable in pressure-proof and gas-tight articles, is disadvantageously high because the degassing is not satisfactory. In addition, the excessive resin sealant which is recovered during hydroextraction is returned to and reused in the sealant impregnation step. However, sealant recovery is not very high. This is because during hydroextraction the resin sealant is not entirely removed but is removed to such an extent that it falls down as droplets. Therefore, the resin sealant which remains in holes or recesses of an intricate workpiece cannot be hydroextracted. The resin sealant which is not hydroextracted is rinsed and becomes waste since it is difficult to separate the resin sealant from water. In the described prior art, one part of the resin sealant is used to impregnate a workpiece while ten or more parts of the resin sealant are not recovered and thus are not reused, with the result that impregnation recovery is low.
According to another prior art, in which an inorganic material is mainly used as a sealant, a workpiece is placed in an impregnation vessel, the pressure of the impregnation vessel is decreased, the sealant is poured into the impregnation vessel, and compressed air pressure is applied to the surface of a sealant bath so as to impregnate the workpiece with the sealant under pressure. The sealant is then returned to a reservoir vessel, and the pressure of the impregnation vessel is reverted to normal. Next, the workpiece is transferred from the impregnation vessel to a rinsing vessel, in which the workpiece is rinsed with water which is stirred with compressed air. This prior art also involves disadvantages resulting from air being dragged into the sealant.